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A postman had to deliver a letter to a house that was surrounded by a five foot wall. The house could be approached only by the main path. Unfortunately, a ferocious dog was tied by a long lead to a nearby tree so that the path was well within the dog s range. If the postman walked up the path, he was sure to be attacked by the dog. How did he outmanoeuvre the dog and deliver the letter?

The postman kept circling the house outside the wall, watching the dog. The dog kept circling the tree, watching the postman. When the dog had wrapped his leash around the tree enough times, the postman could safely go up the main path and deliver the letter.